clean the surface really well. Soap & water might cut it,but when i put decals on my dirtbike, i spray the area with brake cleaner, then wipe and apply the decal. For larger stuff i've heard about spraying a soapy water solution onto the back of the sticker and the surface, then you can slide the sticker where ever you want it, and then you're supposed to take a credit card or something like it and squeejee the water out from under it and it will stick forever...never tried it tho.
~Crock
Ski like you fuck; fast and hard with a stupid grin on your face.